Fix name of RHEL

The company is called Red Hat, and its flagship product is Red Hat Enterprise Linux (commonly abbreviated as RHEL). There is no such thing as RedHat (and never was).

doc/install/requirements.md

@@ -15,7 +15,7 @@ It should also work on (though they are not officially supported):
 - CentOS
 - Fedora
 - Gentoo
-- RedHat
+- RHEL
 ## Other Unix Systems